Biomedical skin electrodes are widely used in a variety of situations, whenever for example it is required to establish an electrical connection between the surface of the body of a patient and external medical equipment for transmission of electrical signals.
Modern medicine uses many medical procedures where electrical signals or currents are received from or delivered to a patient's body. The interface between medical equipment used in these procedures and the skin of the patient is usually some sort of biomedical electrode. Such electrodes typically include a conductor which must be connected electrically to the equipment, and a conductive medium adhered to or otherwise contacting skin of the patient, and they are of varying types with a wide variety of design configurations which will generally depend on their intended use and whether for example they are to be used as transmission electrodes or sensing i.e. monitoring electrodes.
Among the therapeutic procedures using biomedical electrodes are transcutaneous electric nerve stimulation (TENS) devices used for pain management; neuromuscular stimulation (NMS) used for treating conditions such as scoliosis; defibrillation electrodes to dispense electrical energy to a chest cavity of a mammalian patient to defibrillate heart beats of the patient; and dispersive electrodes to receive electrical energy dispensed into an incision made during electrosurgery.
Among diagnostic procedures using biomedical electrodes are monitors of electrical output from body functions, such as electrocardiograms (ECG) for monitoring heart activity and for diagnosing heart abnormalities.
For each diagnostic, therapeutic, or electrosurgical procedure, at least one biomedical electrode having an ionically conductive medium containing an electrolyte is adhered to or is otherwise contacted with mammalian skin at a location of interest and is also electrically connected to electrical diagnostic, therapeutic, or electrosurgical equipment. A critical component of the biomedical electrode is the conductive medium which serves as the interface between the mammalian skin and the diagnostic, therapeutic, or electrosurgical equipment, and which is usually an ionically conductive medium.
Biomedical electrodes are used among other purposes to monitor and diagnose a patient's cardiovascular activity. Diagnostic electrodes are used to monitor the patient immediately and are only applied to the patient for about five to ten minutes. Monitoring electrodes, however, are used on patients in intensive care for up to three days continuously. In contrast, Holter electrodes are used to monitor a patient during strenuous and daily activities.
Although all of the biomedical electrodes just referred to are used to record cardiovascular activity, each electrode requires specific features or characteristics to be successful. Thus, the diagnostic electrode does not have to remain adhered to a patient for extensive periods but it does have to adhere to hairy, oily, dry and wet skin effectively for the five to ten minutes of use. The monitoring electrode has to adhere for a longer period of time although the patient is often immobile during the monitoring period. The Holter electrodes is susceptible to disruption from adhesion due to physical motion, perspiration, water, etc., and therefore requires the best adhesion and at the same time comfort and electrical performance.
In the biomedical electrodes known in the prior art the ionically conductive medium which serves as an interface, between the skin of a mammalian patient and the electrical instrumentation, ranges from conductive gels and creams to conductive pressure sensitive adhesives. However, while the conductive media can be in the form of pressure sensitive conductive adhesives, for monitoring or Holter biomedical electrodes the use of such conductive adhesives is not generally adequate on their own to maintain adhesion to mammalian skin and additional hypoallergenic and hydrophobic pressure sensitive adhesives may be employed around the conductive medium to provide the required mammalian skin adhesion. U.S. Pat. No. 5,012,810 (Strand et al.) and U.S. Pat. Nos. 4,527,087, 4,539,996, 4,554,924 and 4,848,353 (all Engel), the disclosures of which are incorporated herein by reference, are examples of documents that disclose biomedical electrodes which have a hydrophobic pressure sensitive adhesive surrounding the conductive medium.
In general, a desirable skin electrode is one which maintains good electrical contact with the skin and is free of localized current hot spots, i.e. exhibits uniform conductivity. For example, it has been found that a prior art electrode utilizing karaya-gum tends to creep in use and flatten out, exposing skin to possible direct contact with the current distribution member or lead wire. A desirable skin electrode should also usually have a low electrical impedance.
WO-A-95/20634 (3M), the disclosure of which is incorporated herein by reference, describes a bicontinuous pressure sensitive adhesive having a continuous phase of a hydrophobic pressure sensitive adhesive polymer and a continuous phase of a hydrophilic polymer, the adhesive being stated as having utility in biomedical electrodes, mammalian skin coverings and pharmaceutical delivery devices.
WO-A-97/05171 (3M), the disclosure of which is incorporated herein by reference, describes a bicontinuous pressure sensitive adhesive having a continuous phase of a hydrophobic pressure sensitive adhesive polymer and a continuous phase of a hydrophilic polymer, the adhesive having peel adhesion of at least 3 Newtons per 100 mm as measured according to a PSTC-1 Test as defined in the disclosure, the adhesive again being stated as having utility in biomedical electrodes, mammalian skin coverings and pharmaceutical delivery devices.
The adhesives of the above 3M publications are prepared by polymerization of a microemulsion of hydrophobic and hydrophilic monomers, the microemulsion having oil and aqueous phases in which the respective hydrophobic and hydrophilic monomers reside prior to polymerization, optionally with further components such as certain stated surfactants, water soluble additives, other polar monomers etc. In their respective Examples 38, for instance, both 3M publications describe a pressure sensitive adhesive formed from a polymerized microemulsion containing water, hydrophobic monomer (isooctyl acrylate), hydrophilic monomer (2-acrylamido-2-methylpropane sulphonic acid), a hydrophilic reactive oligomer (AM90G ester, which is a polyethylene oxide acrylate), surfactant (SAM 211) to construct the microemulsion, and N-vinyl pyrrolidone (which is a further reactive polar monomer in addition to the hydrophilic monomer), together with a photoinitiator. On suitable irradiation of the microemulsion, the polymerized bicontinuous adhesive is stated to be formed.
WO-A-97/24149 (3M), the disclosure of which is incorporated herein by reference, describes a lipophilic polar pressure sensitive adhesive stated to have enhanced adhesion to greasy skin, the adhesive including a hydrophilic polymer matrix, a polar organic plasticiser and at least 9 wt % of a surfactant having an HLB (hydrophile lipophile balance) value of 10 to 17. It is stated generally that the hydrophilic polymer matrix may be selected from a range of polymers including homo- and copolymers of, for example, (meth)acrylic acid and salts thereof, acrylamide, -vinyl pyrrolidone and acrylamidopropane sulphonic acid and salts thereof. The adhesive is prepared by polymerization in a homogeneous aqueous mixture.
The adhesive disclosed in WO-A-97/24149 is stated to have utility in biomedical electrodes, mammalian skin coverings and pharmaceutical delivery devices. However, only acrylic acid homopolymer and N-vinyl pyrrolidone homopolymer based adhesives are specifically exemplified in the working Examples. The presence of a hydrophobic monomer and/or polymer is not described.
U.S. Pat. No. 5,338,490 (Dietz et al), the disclosure of which is incorporated herein by reference, describes a two-phase composite pressure sensitive adhesive having discontinuous hydrophobic pressure sensitive adhesive domains within a continuous hydrophilic phase. The adhesive is stated to be ionically conductive regardless of the amount of water present, whereby the adhesive is stated to have utility in biomedical electrodes. This ionic conductivity is achieved by the use, as the hydrophilic phase, of an ionic salt dissolved in a solvating polymer generally selected from the group consisting of crosslinked poly(N-vinyl lactam), crosslinked polyacrylamide and its ionic forms, crosslinked polyacrylic acid and its salts, crosslinked poly-(2-acrylamido-2-methylpropane sulphonic acid), its salts, crosslinked copolymers of the acid, crosslinked copolymers of salts of the acid, or mixtures or combinations thereof. The discontinuous hydrophobic domains are formed of hydrophobic materials generally selected from polyacrylates, polyolefins, silicone adhesives, natural or synthetically derived rubber base adhesives, polyvinyl ethers and blends thereof. The adhesive is stated to be prepared by preforming the hydrophobic and hydrophilic components and codissolving them in a solvent, which is then removed by boiling off to leave the adhesive.
The prior art adhesives suffer from a number of disadvantages, whether under performance, manufacturing, economic, environmental or safety considerations, or any combination thereof.